Did you ever want to check which lead of an LED is the anode and which is the cathode or just see how bright it is when on and end up frying your LED in the process? This constant current source LED build allows you to test your LED without needing to choose a current limiting resistor and also provides a visual indication of having the leads reversed.

In other words, we will make it easy for you to test your LED and determine the anode and cathode by providing the plans and schematic needed to test your LED and identify the leads without doing any damage. And then package it in test equipment form so you can use it over and over in the future.

This instructabe is an easy to moderate level of difficulty to complete and should take about 1 to 1 /12 hours to finish. You need to be able to use a drill, cutters, and pliers. The level of soldering is easy. The cost is in the neighborhood of $30 depending in where you purchase the parts.

This instructable is a a collaborative effort of Tom Evans, the circuit designer and build creator, and Bruce Schreiner, creator of the instructions, documentation, and the instructable. We teach electronics to aspiring electronic technicians at Thaddeus Stevens College of Technology (a 2 year AAS degree-granting trade/technical college) located in Lancaster, Pennsylvania - USA.

Step 1: Gather the Parts

Physical Parts

Plastic Project Box, 2.2" x 3.5" x 1"
- Serpac C-Series, C12, 2.40" x 3.75" x 1.00"
Velcro with Tape Backing - both sexes

Electronic Parts

Momentary Contact Single Pole Single Throw Switch (SPST)
- Alco TPF13 SPDT Switch from ??? will work
LM317T Adjustable Voltage Regulator
- ST Microelectronics, LM317T, TO-220 Package
Red LED
- T1 size
2.7V 1/2 Watt Zener Diode
- 1N5223
Red Banana Test Jack
Black Banana Test Jack
120 Ohm, 1/8 Watt Resistor
9V Battery Clip
- Keystone Electronics from Electronix Express

Resources for Identifying and Understanding Electronic Parts

You must know a little about electronics to help in identifying parts. These three books are a great resource when identifying and understanding parts in the electronic world.

Places to Buy Parts

Buying electronic parts is definitely a mail order thing. You must search the catalogs and websites for a while to get used to the organization of the parts before you can identify them.

Step 2: Tools Required

Common tools and equipment needed from around the house or elsewhere:

Cordless Drill, Dremel, or Drill Press
Drill Bits
- Sizes: Depend on exact parts ordered
Soldering Iron
Solder
Electronic diagonal cutters and needle nose pliers
Screwdriver

When buying tools and soldering equipment for the first time it is probably best to spend just a little more and purchase from companies that sell these types of tools. In the electronics area, the two companies below sort through the world of tool choices and provide the most economical items while still being of above acceptable quality.

Step 3: Step 1: Prepare the Project Box and Place the Components

In this step we are going to drill the holes in the project box for the test switch and banana test jacks. We will also attach the Velcro strip for holding the 9V battery clip in the bottom of the project box.

Rather than give exact measurements for these holes and placements we are suggesting that you look at each part and match it to the diagram above to get a rough placement of the parts. Check to make sure that the switch and banana test jacks will not hit anything when being inserted in the drilled holes before you do the drilling. For those of you who need to have everything even and centered (that would be me) feel free to grab your ruler and measure and mark everything.

Drill the Test Switch Hole

Using a 11/32" drill bit, drill a hole on the left side of the bottom of the project box (see picture above) about an 1" down from the top and centered. Eyeball this to see that the switch body will not hit any of the plastic protrusions inside the project box when inserted. Do not insert the switch yet, it might be in the way of other drilling.

Drill LED Test Connection Holes for Banana Test Jacks

Using a 1/2" drill bit, drill two holes in the top side of the bottom of the project box (see picture above) to mount the red and black banana test jacks (black on the left, red on the right when viewing from top). These holes are about 5/8" in from each side and 3/8" up from the bottom of the box. You can remove the mounting nut, place the banana test jacks through the holes, and tighten the nut on the inside to hold the jacks in place. The switch can also be mounted on the left side of the box with the lock washer on the inside and the nut on the outside.

Drill the hole for the T1 LED

Drill a 1/8" hole centered between the banana test jacks on the top side of the bottom of the project box for placing the T1-size red LED. The LED is slid into this hole from the inside and is only secured in place when soldering the leads in future steps.

Cut and Place Velcro for 9V Battery Clip

Cut a 1 1/2" strip of Velcro to place on the bottom of the project box to secure the 9V battery clip. Both sides of the Velcro fastener should be cut and then one side of the adhesive protective strip should be removed and the Velcro should be adhered to the 9V battery clip. Remove the other side of the adhesive protective strip and placed the battery clip into the bottom of the project box. The picture shows the approximate placement.

Step 4: Step 2: Connect the Components

Now it is time to make the connections for all components using a soldering iron and solder. Technicians and engineers would normally solder the electronic components to a printed circuit board (PCB) designed to implement the circuit. These circuit connections would be documented using a schematic.

We are foregoing the PCB and soldering the parts together directly for this project. The documentation for the connections will be the picture of a finished project box from above. Refer to it as you follow the instructions, your finished device must look physically the same as the pictures. Metal parts from one component touching another is not a good thing in electronic circuits unless they are specifically connected in the directions and soldered.

Follow the pictures in order to view examples of what to accomplish. The first picture shows how the parts should be connected when complete and the others step through the process to get there.

Switch Connections

Solder the left test switch lug to -9V terminal as shown above.
Solder a short bare wire to the center test switch lug and to the black banana test jack lug as shown above.

Zener Diode & LED Connections

Identify the cathode side of the Zener diode (the one with the black band). The Zener diode is pictured individually in a previous step if you cannot identify it. The body appears to be clear glass with coloring.
Solder the cathode side of the Zener diode (the one with the black band) to cathode side of the LED. Identifying the cathode side of the LED is a little tricky and must be done correctly. If you look inside the LED the larger LED part is the cathode. Sometimes LEDs can be identified by the length of the leads, but not always and definitely not if someone already cut the leads. The longer lead is the anode if the leads were not altered. Finally, if the LED has a flattened edge the lead by this edge is the cathode. Not meant to confuse, that is just the way it is (here is a link if you need additional assistance: LED Cathode/Anode Identifcation Instructable ).
Solder the anode lead of the Zener diode, which is the only one free, to the black banana test jack lug.
Finally, solder the anode side of the LED, which is the only one free, to the red banana jack socket test lead lug.

Transistor Connection

Bend the leads of the voltage regulator (part with three legs) as shown.
Solder the 120 Ohm resistor to the center lead as show in the next picture. Either side of the resistor works, it doesn't matter.
Position the voltage regulator component in the box exactly as shown and solder the outer lead to the +9V side of the battery clip as shown.
Solder the other outer lead of the voltage regulator to the red banana jack socket test lead lug as shown.
Solder the free side of the resistor to the red banana jack test lead lug as shown.

Step 5: Testing and Use of the LED Tester

Put a 9V battery into the battery clip
Connect an LED to test to the red and black banana jack sockets or via test leads made for the tester. If using test leads connect a black test LED to the black banana jack and a red test lead to the red banana jack.
What you think is the cathode of the LED (large internal part) should be connected to the black lead or jack and the anode of the LED under test (small internal part) should be connected to the red lead or jack.
When connected correctly, push the test button and the LED under test should light as long as your battery is good.

If the LED being tested is connected with the leads reversed, the LED between the banana test jacks on the tester will light indicating that the LED is connected in reverse. The same LED with light if the LED under test does not have both sides connected.

The advantage of the LED tester is that you can determine the tested LED leads (anode and cathode) without needing to choose a current limiting resistor value and risk of damaging your LED.

We like to store our testers and test leads in fancy cigar boxes that can be found in all sizes and shapes at your local smoke shop. We don't smoke (except the inhaling while we are looking for boxes) and many of the smoke shops use the money they charge for empty boxes for good purposes like training for guide dogs.

Step 6: Optional: Operation of the Circuit

Electronic technicians fix a piece of electronic equipment using a schematic which documents the parts used in a circuit and how they are connected. There is a bit of learning and understanding along with various pieces of test equipment like an oscilloscope, logic analyzer, power supply, signal generator, and digital voltmeter (DVM) that allows them to determine what is happening in the circuit. The schematic for the LED tester is presented above. An explanation of the theory and operation of the circuit is provided below for your learning enjoyment!

Theory of Operation

Checking an LED requires two safeguards to avoid damaging the device:

The current flowing through the LED in the forward direction (called forward current) must be limited to a safe level for the device under test (and is different for LED part numbers).
The reverse voltage across the LED must never exceed 5V.

Controlling the Current

The LM317 is a voltage regulator integrated circuit (IC) with three pins entitled input, adjust, and output.
The circuitry inside the LM317 maintains approximately 1.2V between the adjust pin and the output pin.
Placing a 120 ohm resistor between these two pins forces the 120 ohm resistor to have a constant 1.2V across it.
This creates a constant current of 10mA from Ohm's Law (1.2V / 120 Ohms).
The constant current of 10mA (conventional flow) flows from the positive terminal of the 9V battery through the LM317 (entering at the input pin and exiting at the output pin), then through the 120 ohm resistor to the red output banana jack.
The current can continue through the LED under test and return through the black banana jack to teh negative battery terminal.
There is a small amount of current flowing out of the adjust pin of the LM317 but this can be treated as insignificant.

Limiting the Reverse Voltage

A 2.7V zener diode in series with a red LED provides a path that is in parallel with the output of the tester.
This parallel path makes it impossible for the output of the tester to exceed the combined voltage drop of the zener diode and the LED (approximately 4.5V).
If a test LED is connected to the tester in reverse it will appear as an open circuit but will not get more than 4.5V across it.
Whenever the output of the tester is open 10mA of constant current will flow through the parallel zener diode/LED path.

Battery Test

A dead battery in the tester can lead to false conclusions that an LED under test is bad.
The red LED in between the two banana test jacks (in the parallel path) serves as a good battery indicator when no LED is connected to the tester and the test button is pushed.